This invention relates to devices used to measure the curvature of the anterior corneal surface.
The curvature of the anterior corneal surface is of interest in a number of disciplines. For example, in the course of performing surgical procedures on the eye, such as photorefractive keratectomy, the anterior surface of the cornea is modified in order to improve the refractive characteristics of the human eye (i.e., to correct for refractive errors). In the field of contact lens fitting, it is desirable to map the anterior corneal surface so that the posterior surface of the contact lens can be more precisely fitted in order to improve lens performance and reduce eye irritation. Of particular interest in mapping the anterior corneal surface is the curvature of that surface. Attempts to measure this curvature have centered about the technique of reflecting patterns from the surface using optical apparatus with patterns of different types, including bars, squares and, currently, concentric circles or circular segments termed placido rings (mires). Examples of such devices are described in U.S. Pat. No. 4,490,022 for "Apparatus for Corneal Corrective Techniques"; U.S. Pat. No. 4,685,140 for "Keratograph Auto Scanner System"; and U.S. Pat. No. 4,820,039 entitled "Medical Apparatus for Diagnosing Eye Conditions", the disclosures of which are hereby incorporated by reference.
Generally, the preselected pattern, which currently preferably constitutes placido rings, is reflected from the anterior corneal surface and captured either on film or as an electronic video image. After the image is captured, it is analyzed using various techniques to afford both qualitative and quantitative analysis of the corneal surface curvature. The curvature, in turn, can be related to the refractive power of the anterior corneal surface at given points of interest.
While known techniques for analyzing the curvature of the anterior surface of the cornea have been found to be useful, there are certain disadvantages. For example, the curvature analyses in many known systems are made with reference to the vertex of the corneal surface, sometimes termed the central reflex. This point, however, is typically displaced from the actual center of the pupil, from which the measurements should ideally be taken. In addition, the known technique using placido rings is difficult to relate to abrupt changes in the surface curvature. Also, the repeatability of results with many existing systems is on the order of 0.5 diopter, which is less than desirable.